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and Bimetallic PtOx, PdOx, PtPdOx Clusters on CO Sensing ... › publication › fulltext › Influence-... › publication › fulltext › Influence-...by P Kutukov · Cited by 10 — synergistic effect—i.e., that the activity of the bimetallic catalyst exceeds the su

Influence of Mono- and Bimetallic PtOx, PdOx, PtPdOx Clusters on CO Sensing by SnO2 Based Gas Sensors Pavel Kutukov 1 , Marina Rumyantseva 1, * , Valeriy Krivetskiy 1 , Darya Filatova 1 , Maria Batuk 2 , Joke Hadermann 2 , Nikolay Khmelevsky 3 , Anatoly Aksenenko 3 and Alexander Gaskov 1, * 1 2 3

*

Chemistry Department, Moscow State University, 119991 Moscow, Russia; [email protected] (P.K.); [email protected] (V.K.); [email protected] (D.F.) EMAT, University of Antwerp, B-2020 Antwerp, Belgium; [email protected] (M.B.); [email protected] (J.H.) LISM, Moscow State Technological University Stankin, 127055 Moscow, Russia; [email protected] (N.K.); [email protected] (A.A.) Correspondence: [email protected] (M.R.); [email protected] (A.G.); Tel.: +7-495-939-5471 (M.R. & A.G.)

Received: 17 October 2018; Accepted: 3 November 2018; Published: 7 November 2018

 

Abstract: To obtain a nanocrystalline SnO2 matrix and mono- and bimetallic nanocomposites SnO2 /Pd, SnO2 /Pt, and SnO2 /PtPd, a flame spray pyrolysis with subsequent impregnation was used. The materials were characterized using X-ray diffraction (XRD), a single-point BET method, transmission electron microscopy (TEM), and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) with energy dispersive X-ray (EDX) mapping. The electronic state of the metals in mono- and bimetallic clusters was determined using X-ray photoelectron spectroscopy (XPS). The active surface sites were investigated using the Fourier Transform infrared spectroscopy (FTIR) and thermo-programmed reduction with hydrogen (TPR-H2 ) methods. The sensor response of blank SnO2 and nanocomposites had a carbon monoxide (CO) level of 6.7 ppm and was determined in the temperature range 60–300 ◦ C in dry (Relative Humidity (RH) = 0%) and humid (RH = 20%) air. The sensor properties of the mono- and bimetallic nanocomposites were analyzed on the basis of information on the electronic state, the distribution of modifiers in SnO2 matrix, and active surface centers. For SnO2 /PtPd, the combined effect of the modifiers on the electrophysical properties of SnO2 explained the inversion of sensor response from n- to p-types observed in dry conditions. Keywords: nanocrystalline semiconductor oxides; nanocomposites; tin oxide; platinum; palladium; bimetallic particles; carbon monoxide; gas sensor; response inversion

1. Introduction Because SnO2 is a wide-bandgap oxygen-deficient n-type semiconductor with optical transparency, electron conductivity, and a high specific surface area, it is suitable for a large range of applications, including in solar cells, as catalytic support, and as solid state gas sensors [1]. However, the use of bare SnO2 is often limited by lack of selectivity and a high operating temperature [2]. Chemical modification is a well-established practice intended to solve those problems [2–5]. This involves the creation of new active sites, with specific adsorptivity and reactivity toward target gases (i.e., carbon monoxide), on the surface of a semiconductor matrix. Carbon monoxide (CO) is a colorless, odorless, and tasteless toxic gas, produced by automotive emissions, natural gas manufacturing, industrial activities, and the incomplete burning of fuels [6]. In the Nanomaterials 2018, 8, 917; doi:10.3390/nano8110917

www.mdpi.com/journal/nanomaterials

Nanomaterials 2018, 8, 917

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human body, it reacts readily with hemoglobin to form carboxyhemoglobin. Carbon monoxide exposure is still one of the leading causes of unintentional and suicidal poisonings, and it causes a large number of deaths annually. [7] Thus, real-time monitoring of CO is extremely important for safety reasons. Carbon monoxide is a reducing gas without pronounced acid or basic properties [3]. To enhance the sensor signal of a SnO2 based sensor toward such gases, the catalytically active additives metallic platinum, gold, and silver are the most effective [6,8]. These modifiers take part in the oxidation of CO on the surface of the semiconductor oxide and lead to a change in the type and concentration of active groups on that surface [5]. This not only leads to an increased sensor response, but also to a decrease in temperature, at which a maximum sensitivity is observed [2–5]. The selectivity of heterogeneous catalysts of oxidative processes is determined by the energy of adsorption of the reducing gas, the binding energy with surface oxygen (which is an oxidizer) and binding energies with intermediates and reaction products. In CO oxidation, the optimal catalysts are palladium (Pd) and platinum (Pt), since the energy of chemisorption of oxygen on the clusters of these metals (340–360 kJ/mol) is close to the binding energy of CO with their surface [9,10]. Palladium and platinum are some of the most effective modifiers for improving the sensor properties of semiconductor metal oxides in CO detection [11–27]. The properties of bimetallic catalysts are now being actively investigated due to the expected synergistic effect—i.e., tha

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